Method for the production of plane-parallel platelets by using organic separating agents

ABSTRACT

A new method is proposed for the production of plane-parallel platelets, comprising the steps a) vapor-deposition, at a pressure below atmospheric pressure, of a separating agent onto a carrier to produce a separating agent layer, b) vapor-deposition, at a pressure below atmospheric pressure, of at least one product layer onto the separating agent layer, c) dissolution of the separating agent layer in a solvent and production of a suspension in which the at least one product layer is present in the form of plane-parallel platelets, in which method the separating agent is selected from the group consisting of anthracene, anthraquinone, acetamidophenol, acetylsalicylic acid, camphoric anhydride, benzimidazole, benzene-1,2,4-tricarboxylic acid, biphenyl-2,2-dicarboxylic acid, bis(4-hydroxyphenyl)-sulfone, dihydroxyanthraquinone, hydantoin, 3-hydroxybenzoic acid, 8-hydroxyquinoline-5-sulfonic acid monohydrate, 4-hydroxycoumarin, 7-hydroxycoumarin, 3-hydroxynaphthalene-2-carboxylic acid, isophthalic acid, 4,4-methylene-bis-3-hydroxynaphthalene-2-carboxylic acid, naphthalene-1,8-dicarboxylic anhydride, phthalimide and its potassium salt, phenol-phthalein, phenothiazine, saccharin and its salts, tetraphenylmethane, triphenylene, triphenylmethanol, and also mixtures of at least two of those substances. Separating agents that have been found to be especially suitable are aromatic compounds having at least one benzene ring. The present invention relates also to the use of the plane-parallel platelets thereby produced and also to the products obtainable using such methods.

The present invention relates to a method for the production ofplane-parallel platelets, to the use of the plane-parallel platelets soproduced and also to products obtainable using such methods.

There is great interest in plane-parallel platelets or “flakes”, usuallyof aluminium or of combinations of metals with oxides, for use aspigments in surface-coating compositions and printing inks, as catalystmaterials, as starting materials for magnetic and electrical screens andas starting materials for conductive surface-coating compositions. Theseproducts differ from conventional pigments, which are produced using agrinding procedure and which are spheres that have been flattened off toa greater or lesser extent and therefore have a substantially lessfavourable surface-to-weight ratio.

Such plane-parallel platelets are typically from 30 to 500 nm thick; theplanar dimensions, that is to say the lengths of the edges, aretypically from 5 to 50 μm.

The pigment content of a metallic automotive finish comprises about80–100 g of aluminium in the case of conventional ground pigmentswhereas just 4–5 g suffice for the same area when plane-parallelplatelets of aluminium produced by the PVD (“physical vapourdeposition”) method are used. Plane-parallel platelets are accordinglydistinguished by greater economy. Just 3–4 layers of such platelets,used in weights of only from 0.3 to 0.4 g/m², produce a layer thatprovides optical coverage of the background.

In accordance with DE 198 44 357 C2 (Weinert), such plane-parallelplatelets (flakes) are produced, in a continuous method, byvapour-deposition of a separating agent layer and a product layer, forexample an aluminium layer, in succession, under the same high vacuum,onto an endless carrier whilst passing through, followed by dissolutionof the separating agent in a subsequent bath of a solvent, which islikewise under vacuum. As a result, the product layer breaks up intoindividual flakes, which are then present in a suspension comprising thesolvent, the dissolved separating agent and the flakes. The suspensionis collected and the plane-parallel platelets are isolated using methodsknown per se and further processed to produce surface-coatingcompositions or printing inks.

A further, discontinuous multi-step method is used for the production ofoptically variable pigment platelets, such as are used for increasingthe security of bank notes against forgery (EP 227 423). U.S. Pat. No.5,278,590 describes a similar method. In U.S. Pat. No. 4,168,985(Venis), U.S. Pat. No. 3,123,489 (Bolomey et al.) and U.S. Pat. No.5,156,720 (Rosenfeld), the separating agents used are inorganic salts,which are dissolved in a subsequent step using water as solvent, as aresult of which the product layer is present in the form of flakes inaqueous suspension.

According to WO 99/65618 (Kittler), waxy substances are vaporised andthen, under the same vacuum, the product layer is vapour-deposited orsputtered. After a large number of revolutions of the carrier, usually arotating cylinder, the arrangement of n layers (wax/metal) is scrapedoff. In a further step, outside the vacuum apparatus, the wax is washedout of the collected paste by means of solvent(s). In all cases, largeamounts of solvent(s), which have to be either reprocessed or disposedof subsequently, are required in order to wash the product.

For the production of plane-parallel platelets of metals such asaluminium, iron, copper and titanium and of water-soluble compounds,salts are not suitable as separating agents. The large surface areas ofsuch platelets react with the water, with formation of hydrogen, or thewater-soluble compounds are simply dissolved. Moreover, the metalsurfaces of such platelets are attacked by the chemical reaction andlose their surface quality. In the case of copper, the anion of thedissolved salt reacts with the metal.

In contrast, in the case of platelets of quartz or titanium dioxideproduced by the PVD method, no reaction occurs; salts are suitableseparating agents. Sodium chloride, sodium tetraborate (U.S. Pat. No.3,123,489; Bolomey et al.) and also sodium fluoride (U.S. Pat. No.4,168,985; Venis) and other salts are known as substances that canreadily be vaporised and that do not undergo decomposition in the PVDmethod. They are, however, not suitable for use when the product layeris to be a metal and/or when water cannot be used as solvent.

Organic compounds that can be evaporated in vacuo substantially withoutundergoing decomposition would be indicated in such cases. From theliterature there are known a number of publications according to whichsuch organic substances can be vapour-deposited, in the form oftransparent, electrically insulating layers, onto a carrier using thePVD method (see, for example, U.S. Pat. No. 3,379,803; Tittmann et al.).

In that case, polymer layers are produced by means of vaporisation ofxylylene compounds. Those compounds form polymers known by the groupdesignation parylenes after condensation, under the influence ofhigh-energy electrons or short-wave ultraviolet light, on a carrier. Thelayers so obtained are not suitable as separating agent layers, thefunction of which is to dissolve rapidly in solvents. Such layers arerather, according to Römpp, Chemielexikon, Volume 4, page 3130,extremely resistant to solvents even at 400° C. and are used especiallyas so-called barrier layers in semi-conductor production.

Further examples of the vaporisability of organic substances for suchlayers by the PVD method are described in U.S. Pat. No. 6,101,316(Nagashima et al.), DE-OS 2 706 392 (Ikeda et al.), DE-OS 2 009 080(Davies et al.) and U.S. Pat. No. 3,547,683 (Williams, Hayes).

According to those publications, addition polymers and condensationpolymers, silicone resins, phthalocyanine dyes and even naturalmaterials such as colophony are vaporised. A further method by means ofwhich organic polymer layers are produced using the PVD method isdescribed in U.S. Pat. No. 5,440,446 (Shaw), wherein a liquid monomer isvaporised, condensed in wet form on a passing film carrier on a cooledroller and, on the same roller, immediately polymerised by electron beambombardment, as a result of which a solid film forms. Subsequently, ametal layer, usually aluminium, is vapour-deposited.

U.S. Pat. No. 4,382,985 discloses the deposition of a polymer film ontoa substrate by means of plasma polymerisation of fluoroalkyl acrylatemonomers. From U.S. Pat. No. 5,904,958 it is known to deposit organicmonomers on substrates by means of vacuum methods and subsequently tocarry out polymerisation. From JP 11-140 626 A (Patent Abstracts ofJapan) it is known to apply a thin film of triazine monomers to asubstrate, for example by means of a vacuum method, and then to carryout polymerisation.

The aim of all those methods is to produce firmly adherent protectivelayers. Rapid solubility in solvents is not desired and would even bedamaging.

Finally, DE 199 33 230 A1 and DE 199 35 181 A1 (Moosheimer et al.)disclose release layers or protective layers comprising organic monomersthat are preferably water-soluble, especially triazine monomers. Suchlayers can be dissolved away using warm water, which is, however, notsuitable for the method according to the invention because of itsreactivity with the metals used according to the invention (evolution ofH₂), because of its residual oxygen content (creeping oxidation) andbecause of the difficulty of removing it from the products.

The problem of the present invention was accordingly to make available asubstantially improved method, compared to the above-mentioned prior artsuch as, for example, DE 198 44 357 C2 (Weinert), for the production ofplane-parallel platelets by using the PVD method.

Suitable separating agents should be capable preferably of being used ina continuous PVD method and especially of being vaporised in anindustrial context in amounts of more than 1 kg/h with little thermaldecomposition. The amounts of non-condensable cracked gases that formshould be substantially less than the capacities of the high-vacuumpumps customarily used for such methods.

The separating agents should be condensable in the form of an amorphouslayer at from 0° to about 50° C., preferably at room temperature, on amoving carrier passing by continuously. They should react as little aspossible with a product layer vapour-deposited in accordance with theinvention onto the separating agent layer, which product layer comprisesmetals such as, for example, aluminium, iron, copper, silver, zinc, tin,titanium and also mixtures thereof, fluorides, such as magnesiumfluoride, or sulfides, such as zinc sulfide, oxides, such as silicondioxide and titanium dioxide, or with multi-layer systems comprisingsuch substances.

The separating agent layer between the carrier and the product layer,from which the plane-parallel platelets are obtained, should be capableof dissolving as quickly as possible. Also, the solvent required fordissolution of the separating agent layer must not react chemically withthe product layer, which then breaks up into fine flakes. The timeavailable is determined by the maximum dwell time in the dissolutionzone. In the case of industrial carrier speeds of from 50 to 250 m/min,this time is typically from 5 to 20 seconds, especially from 5 to 10seconds.

The problem has been solved, surprisingly, by the method of theinvention according to claim 1, the uses of the invention according toclaims 11 to 14 and the product of the invention according to claim 15.The dependent claims relate to preferred embodiments.

The present invention accordingly relates to a method for the productionof plane-parallel platelets, comprising the steps a) vapour-deposition,at a pressure below atmospheric pressure, of a separating agent onto acarrier to produce a separating agent layer, b) vapour-deposition, at apressure below atmospheric pressure, of at least one product layer ontothe separating agent layer, and c) dissolution of the separating agentlayer in a solvent and production of a suspension in which the at leastone product layer is present in the form of plane-parallel platelets, inwhich method the separating agent is selected from the group consistingof anthracene, anthraquinone, acetamidophenol, acetylsalicylic acid,camphoric anhydride, benzimidazole, benzene-1,2,4-tricarboxylic acid,biphenyl-2,2-dicarboxylic acid, bis(4-hydroxyphenyl)sulfone,dihydroxyanthraquinone, hydantoin, 3-hydroxybenzoic acid,8-hydroxyquinoline-5-sulfonic acid monohydrate, 4-hydroxycoumarin,7-hydroxycoumarin, 3-hydroxynaphthalene-2-carboxylic acid, isophthalicacid, 4,4-methylene-bis-3-hydroxy-naphthalene-2-carboxylic acid,naphthalene-1,8-dicarboxylic anhydride, phthalimide and its potassiumsalt, phenolphthalein, phenothiazine, saccharin and its salts,tetraphenylmethane, triphenylene, triphenylmethanol, and also mixturesof at least two of those substances.

The above-mentioned separating agents meet the following conditions:

They are solid, non-polymerisable organic compounds having vapourpressures of less than 10⁻³ Pa at 25° C. (fundamental requirement inorder to be able to use a material without self-vaporisation at roomtemperature in a vacuum of <0.1 Pa).

The separating agent layer is rapidly soluble in industrial solventssuch as, for example, isopropanol, ethyl acetate, butanol, ethanol,petroleum spirit, methyl isobutyl ketone, methyl ethyl ketone orperchloroethylene.

Below their melting points, the separating agents have vapour pressuresof from 10 to 1000 Pa. As a result, use of the separating agentsaccording to the invention results in sublimative vaporisation below thetriple point of the substances and avoids technically disadvantageousspatter formation.

The separating agents have, moreover, high thermal stability and areavailable in industrial amounts for the planned implementation of aproduction process. They have an advantageous price ratio with respectto the metal or product from which the plane-parallel platelets are tobe produced.

Especially suitable for carrying out the method according to theinvention are those of the above-mentioned separating agents which havearomatic structural elements, preferably of the homocyclic type, in themolecule, especially aromatic compounds having at least one benzenering.

In addition, these substances condense in amorphous form. This isimportant for obtaining reflecting metal layers that are to bevapour-deposited onto the separating agent layer. Layersvapour-deposited by the PVD method exactly reproduce the structure ofthe underlying layer, because they do not have a levelling effect likethat of layers applied in liquid form, the surface tension of whichproduces such an effect.

The present invention will be explained in greater detail by means ofthe following Examples, in which preferred embodiments are illustrated,with reference to the accompanying drawing.

FIG. 1 shows a test set-up for implementation of the method according tothe invention. The arrangement according to FIG. 1 was found to benecessary because, in vaporisation tests using substances for whichthere is no data relating to their behaviour in a PVD method, it was tobe expected that unknown cleavage substances would occur. The followinglist gives a description of the individual components of the testingsystem according to FIG. 1.

Chamber Vacuum chamber measuring 300 × 300 × 300 mm having an integralthermal vaporiser, controllable from 20 to 500° C., a liquid nitrogentrap for condensing cracked gases, valves, fitted electrical valves andalso a vacuum- measuring device (not shown) V1 Pump valve Vdr + Throttlevalve, adjustable by means of the throttle D1.8 port D1.8, for throttledpump operation during vaporisation (minimising the intake of crackingproducts into the vacuum pump) Pressure Safety indicator in the event ofthermal decomposition of the switch separating agent under test or apressure increase in the chamber of more than 1.1 × 10⁵ Pa (triggeringevacuation via the absorption column with simultaneous inlet of nitrogeninto the chamber) VN₂ Nitrogen inlet valve for emergencies, that is tosay a pressure of > 1.1 × 10⁵ Pa S1 Non-return valve; the line leads tothe absorption column D65B 2-step rotary vane pump set comprising turbopump; final vacuum better than 10⁻⁴ Pa without pre-filter or 10⁻³ withpre-filter V10 Absorption column by-pass valve for direct evacuation ofthe cold chamber V11 Passes evacuated gas through the absorption columnwhen V10 is closed V13 Manual valve, supplying fresh absorption agentV14 Discharge of used absorption agent

EXAMPLE 1

A selected separating agent, in an amount of 20 grams, was filled into aheatable crucible having an opening of 10×1 cm in the vacuum chamberaccording to FIG. 1. The crucible was indirectly heated by means of ametal grating arranged above it, made from chromium nickel 80Ni20Cr, inthe direct flow path. A sample plate measuring 250×250×1 mm, as thecondensation surface, was fastened to the roof of the chamber. Thechamber was evacuated to 10⁻³ Pa and the metal grating was slowly heatedelectrically to a temperature 30° C. below the known melting point ofthe substance, thereby ensuring that only sublimative vaporisation waspossible. During heating of the crucible by means of a controllablevoltage source, vacuum measurement was carried out. If a collapse in thevacuum to more than 1 Pa was observed, the temperature was lowered and,after 5 minutes, was again increased to 30° C. below the melting point.By that means it was possible to eliminate gas emission effects beforethe measurement. If the vacuum again collapsed to more than 10⁻¹ Pa, theconclusion had to be drawn that decomposition was present to a greateror lesser extent. After each test, the chamber was purged with nitrogen,and the sample plate was removed and weighed. The vapour-deposition ratecould be calculated from the increase in weight and the vaporisationtime. In order for a separating agent to be used economically in anindustrial method, it should satisfy the following condition at asublimation temperature of 30° C. below the melting point:1/A*dm/dt≧0.15 g min⁻¹ cm⁻¹wherein:

-   A is the area from which vaporisation takes place (cm²) and dm/dt is    the vaporisation rate (g/sec).

Vaporisation from the liquid phase was to be avoided because of the riskof spatter and foam formation caused by cracked gases that form. Thevalue of dm/dt≧0.15 g min⁻¹ cm⁻¹ corresponds approximately to thevaporisation rate from a conventional aluminium vaporiser at about 1450°C.

EXAMPLE 2

Under the same vacuum, a selected separating agent and, subsequently, alayer of aluminium were vapour-deposited onto a sample plate. The layerthicknesses were about 40 nm for the separating agent layer and about 45nm for the aluminium layer. The condensation rate was adjusted to 20–30nm/sec by means of the temperature of the vaporisation source. Lowercondensation rates are of little interest because, in a continuousindustrial process, they would result in unfeasibly slow carrier speeds.The measurement was carried out using a known technique by means of aquartz resonator, which was mounted at the same level as the sampleplate in the middle thereof. For that purpose, a hole corresponding tothe diameter of the measurement head was drilled through the middle ofthe sample plate, thereby ensuring that the layer on top of thevibrating quartz was the same as that on top of the sample plate. As aresult of this testing step, it was possible to limit the number ofseparating agents that are suitable for use because, in the case of manyorganic substances, it was found that either they did not condense onthe carrier surface in the form of an amorphous layer or they reactedwith the aluminium vapour-deposited thereon, resulting in matt, milkyand also, in many cases, yellow-coloured layers which, on analysis byESCA using a known technique, were found to be aluminium carbides. Anindication of thermal cracking of some substances was the formation ofan odour, which was investigated after opening the vacuum chamber.

EXAMPLE 3

The apparatus consisted of an immersion bath, in which there wasimmersed the sample plate, which was weighed before and afterdissolution. As a result, it was possible to determine the amountcondensed. The separating agent layer and the vapour-deposited metallayer were dissolved away from the carrier whilst measuring the timetaken for the metal layer to come away and break up into flakes. Thesolvents used in the test series were: isopropanol, ethyl acetate,butanol, ethanol, petroleum spirit, methyl isobutyl ketone and methylethyl ketone. Using this method, it was possible in each case todetermine the fastest solvent for a particular separating agent.

Table I shows the results for the substances tested in Examples 1 to 3with regard to their suitability as separating agents in the methodaccording to the invention.

The vaporisation of polymers is known from the literature and isdifficult to explain chemically. It is not possible for whole chains tovaporise. Rather they are broken down into fragments which, on acondensation surface under a high vacuum, immediately undergo partialre-polymerisation. IR spectroscopy and also further chemical testingshows that the condensate is not the same substance as was present inthe vaporiser. The molecular weight of the condensate is generally70–40% lower. When in the form of a thin layer of from 30 to 100 nm,polymers, which are generally insoluble in isopropanol, ethanol andpetroleum spirit, nevertheless dissolve in such solvents under certainconditions.

Nevertheless, unlike the non-polymerisable separating agents accordingto the invention, polymers have been found to be generally unsuitable aswill be further explained hereinbelow.

From the chemical substances listed under nos. 101 to 228 it waspossible to select types suitable as separating agents that can bevaporised by the PVD method according to the invention. Some condense incrystalline form, depending on the temperature of the substrate, andgive the metal layer condensed on top of them a satin-like appearancewhich, in the case of metal pigments, may even be desirable in somecases.

The disadvantages in using polymers are based on the fact that thepolymers cannot be vaporised satisfactorily, that it is difficult tomaintain a constant vaporisation rate, that the polymers can crack andresult in substantial collapse of the vacuum, that the polymer layerscannot readily be dissolved (in some cases they dissolve unacceptablyslowly), that carbonised residues form in the region of the vaporiser,sublimation is not possible and spatter formation may occur.

It was also found that separating agents that dissolve sufficientlyrapidly between a carrier and a vapour-deposited metal layer aresuitable for a continuous method for the production of plane-parallelplatelets by the PVD method. Especially suitable are those substancesmarked “<5 sec.” and “1” in Table I, which exhibit little thermaldecomposition on vaporisation in vacuo and thereafter exhibit rapidsolubility in organic solvents. Separating agents found to be especiallysuitable were aromatic compounds having at least one benzene ring. (Inthe case of industrial belt speeds for the carrier of about 200 metresper minute and a practicable dissolution zone of max. 30 metres, thereis a maximum of 6 seconds available for the organic separating agent tobe dissolved away. It is of course technically possible for the lengthof such a dissolution zone to be increased or for the dissolution to beassisted by means of mechanical aids.)

It is furthermore advantageous for the separating agent or agents to bebrought into exclusive contact with a heat-resistant ceramic solid, forthe latter to be admixed in the form of powder or granules, and for themixture to be heated in vacuo up to the vaporisation temperaturerequired for the separating agent. It has been found that, in the caseof some separating agents according to the invention, chemical orcatalytic decomposition can be substantially reduced by the lack ofcontact with hot metal surfaces.

This invention opens up the possibility of producing layers of metal,such as aluminium, iron, titanium, copper and others, vapour-depositedby the PVD method, in a continuous method operating in a closed circuit,with a dwell time of limited length in the dissolution zone.

To the person acquainted with the subject matter, it will be obvious andpossible to use derivatives and mixtures of the substances mentioned inTable I as separating agents in order to obtain the desired successfuloutcome. There are, likewise, known variants of the PVD vaporisationtechnique in which two or more substances are vaporised from the samesource or from two or more overlapping sources. It is furthermorepossible for organic separating agents to be fed in continuously to thevaporisation source.

A further variant comprises applying a succession of separating agent,product layer, separating agent, product layer etc., one after another,from 4 or more vaporiser sources in one pass before they aresubsequently all detached together by dissolution in one solvent.

TABLE I Vacuum Metal Dissolution Vaporisation collapse** No. Separatingagent surface time* time (note) (note) 101 polyamide (DELRIN ™)glossy >20 sec. 270° C. 3 (caustic odour in (unsuitable) the vacuumchamber after opening) 102 polystyrene dis- >20 sec. 240° C. 3 coloured(styrene odour) (unsuitable) 103 polyester (polyethylene glossy >20 sec.305° C. 3 terephthalate; PET) (caustic odour) (unsuitable) 104polycarbonate (PC) glossy >20 sec. 300° C. 3 (LEXAN ™) (slight odour)(unsuitable) 105 polymethacrylate (PMMA) glossy >20 sec. 265° C. 3(caustic odour) (unsuitable) 106 polytetrafluoroethylene satin-like >50sec. 420° C. 3 (PTFE) (acrid smell; (unsuitable) discontinued) 107polyimide (Kapton ™) dis- >50 sec. 450° C. 2 coloured (odour of medium(unsuitable) strength) 108 poyvinylcarbazole glossy >20 sec. 240° C. 2CAS No. 25067-59-8 (slight odour) (unsuitable) 109 parylene glossy no360° C. 2 dissolution (expensive and unsuitable) 201 saccharin sodiumsalt yellow <5 sec. 190° C. 2 CAS No. 82325-42-0 discolor- ation 202tetraphenylmethane satin-like <5 sec. 255° C. 1 CAS No. 630-76-2(expensive) 203 triphenylmethanol satin-like <5 sec. 135° C. 1 CAS No.76-84-6 (expensive) 204 phenolphthalein glossy 5–10 sec. 230° C. 2 CASNo. 77-09-8 205 triphenylene glossy 5–10 sec. 170° C. 1 CAS No. 217-59-4206 acetamidophenol matt >10 sec. 120° C. 2 CAS No. 621-42-1 207acetylsalicylic acid glossy <5 sec. 110° C. 1 CAS No. 50-78-2 208camphoric anhydride glossy, 5–10 sec. 180° C. 2 CAS No. 13429-83-9slightly yellow 209 phenothiazine glossy 5–10 sec. 160° C. 1 CAS No.92-84-2 210 anthracene glossy 5–10 sec. 120° C. 3 CAS No. 120-12-7 211anthraquinone glossy 5–10 sec. 250° C. 2 CAS No. 84-65-1 212 mixture of201 and 205 matt 5–10 sec. 170° C. 2 yellow 213 benzimidazole matt 5–10sec. 140° C. 2 CAS No. 51-17-2 214 benzene-1,2,4-tricarboxylic yellowish<5 sec. 210° C. 1 acid CAS No. 528-44-9 215 biphenyl-2,2-dicarboxylicglossy <5 sec. 190° C. 1 acid CAS No. 482-05-3 216bis(4-hydroxyphenyl)sulfone yellowish 5–10 sec. (substantial 3 CAS No.80-09-1 decomposition) 217 dihydroxyanthraquinone matt 5–10 sec. 160° C.3 CAS No. 81-64-1 218 hydantoin glossy 5–10 sec. 190° C. 2 CAS No.461-72-3 219 3-hydroxybenzoic acid yellowish <5 sec. 170° C. 3 CAS No.99-96-7 220 8-hydroxyquinoline-5- yellowish 5–10 sec. 280° C. 3 sulfonicacid monohydrate CAS No. 84-88-8 221 4-hydroxycoumarin glossy 5–10 sec.180° C. 2 CAS No. 1076-38-6 222 7-hydroxycoumarin glossy 5–10 sec. 180°C. 2 CAS No. 93-35-6 223 3-hydroxynaphthalene-2- glossy 5–10 sec. 170°C. 3 carboxylic acid CAS No. 86-48-6 224 isophthalic acid glossy 5–10sec. 270° C. 2 CAS No. 121-91-5 225 4,4-methylene-bis-3- matt 5–10 sec.250° C. 2 hydroxynaphthalene-2- carboxylic acid CAS No. 130-85-8 226naphthalene-1,8- glossy <5 sec. 220° C. 1 dicarboxylic anhydride CAS No.81-84-5 227 phthalimide glossy <5 sec. 190° C. 2 CAS No. 85-41-6*relative time under identical test conditions by means of immersion at25° C. in the fastest-acting solvent selected from the group consistingof isopropanol, ethyl acetate, ethanol, petroleum spirit, methylisobutyl ketone and methyl ethyl ketone **a pressure increase 7 × 10⁻² 2× 10⁻¹ >2 × 10⁻¹ [Pa] of up to: corresponds to a rating of 1: slight 2:moderate 3: substantial This increase is dependent upon the testapparatus used and the installed suction power of the vacuum pumps. Itis given here as a reference value.

The plane-parallel platelets produced by the method according to theinvention have further advantages compared to pigments according to theprior art, especially in respect of their further processibility.

Metal pigments of aluminium according to the prior art already beingfurther coated on a large industrial scale by a CVD method all originatefrom grinding processes, which are carried out in ball mills with theaddition of test petroleum spirit and stearates. The removal of suchresidual layers is laborious, is not entirely possible and requiresseveral washing procedures using clean solvent in a cascade arrangement.An example which uses ground aluminium as basic material is described inEP 0 708 154 (Schmid and Mronga). As a result of CVD coating with SiO₂and an absorbing layer, an interference pigment is produced.

Removal of organic protective layers by vaporisation in the case ofsemi-conductor applications is described in DE 199 35 181 C2(Moosheimer). These layers are also referred to as masks which can beremoved by vaporisation.

According to the prior art, inorganic salts are vapour-deposited, forexample according to U.S. Pat. No. 3,123,489 (Bolomey et al.), asseparating agents for producing layers of zinc sulfide, fluorides oroxides. U.S. Pat. No. 6,270,840 (Weinert) uses likewise vapour-depositedsalt layers as separating agents in a continuous method, in which allsteps take place at the same time in vacuo. Although, in water, saltsdissolve away from the surfaces of the plane-parallel plateletscompletely, they have the disadvantage that, in the case of plateletsproduced from copper, silver or tin, which can be used as conductivesurface-coatings, they result in rapid chemical corrosion of thosemetals in the dissolution bath of water, especially because thevaporised salts, once dissolved, give rise to anions.

Compared to the prior art of the coating of plastics films, subsequentvapour-deposition of metal layers or combinations of metal, oxide orfluoride layers by the PVD method (for example, chromium, silicon oxideor magnesium fluoride) and dissolution of the coating layer in anorganic solvent to produce plane-parallel platelets of such materials,use of the separating agents according to the invention has thesubstantial advantage that they do not leave behind polymers oroligomers attached to the product layers. Rather, they leave behind theseparating agents in a monomolecular layer.

In contrast, polymers attached to the surfaces of plane-parallelplatelets, whose metal layer is typically only 40 nm thick, formdeposits of about from 10 to 15 nm twice, that is to say from 50 to 75%by volume. Because of their substantially smaller molecule size, theseparating agent layers according to the invention leave behind residuesof only about from 10 to 15% by volume.

Usually the plane-parallel platelets produced by the method according tothe invention are immediately further processed in the form of thesuspension that is obtained. It is, however, also possible for theplane-parallel platelets produced by the method according to theinvention to be further cleaned.

In a subsequent step, the remaining monomolecular layers can be removedso that no residues remain, by vaporising them off from the producedplane-parallel platelets under a vacuum of from 10⁻³ to 10 Pa so thatsubstantially no residues remain. The separating agent vaporised off isdeposited as a solid on cold condenser surfaces and removed. Thevaporisation temperatures during the procedure are lower than 350° C.,preferably lower than 300° C.

Plane-parallel metallic platelets cleaned in that manner, withoutelectrically insulating layers on their surfaces, are suitable, forexample, for producing conductive surface-coating compositions, that isto say electrically conductive dispersions. By virtue of their smallparticle size, however, such platelets are also suitable for applicationprocesses by means of inkjet printing, intaglio printing andflexographic printing. Also, establishing contact with connection siteson electronic components or photovoltaic components is substantiallysimplified as a result.

This procedure would not be possible if adhering oligomers or polymerswere present; they would undergo thermal decomposition and theircracking products would result in substantial discoloration of theplane-parallel platelets. As a result, the metallic appearance and theelectrical conductivity would, at least in part, be lost.

The phrase “so that no residues remain”, which is used above, is definedherein to mean that a cohesive monomolecular layer of the organicseparating agent is no longer present on the surface. This requires theaverage theoretical layer thickness of contaminants caused by separatingagent to be less than about 0.5 nm, corresponding to a “residue” of max.0.6 mg per m² when the density of the separating agent is about 1.2g/cm³. The surfaces are then covered by non-cohesive islands only.

An example of a suitable measurement procedure comprises heating anamount of 100 g of dry aluminium platelets that are 40 nm thick,corresponding to a calculated surface area of 1000 m², to about 400° C.in a vacuum chamber which is heated on all sides, and measuring thepressure increase caused by the gaseous decomposition products. Acomparison measurement of the pressure increase using 0.6 gram of thesame separating agent on its own will give a calibration value. It isassumed in such a determination that the separating agents on which thisinvention is based will be thermally decomposed into gaseous componentscompletely, without leaving behind cracking products, which is indeedthe case.

Further processing of the resulting thermally cleaned plane-parallelplatelets into printing inks, spray paints or dry powder coatings istherefore much more practicable. It is not necessary to take intoaccount the chemical acceptability of organic residues because these,having been vaporised off, are no longer present to any appreciableextent.

The plane-parallel platelets produced by the PVD method and madeavailable in the above manner so that they are free of organic residuesand dry, can then be coated with other substances in a further stepcarried out after comminution and grading of the platelets by size usingmethods known per se, without organic residual layers having an adverseeffect on the further coating.

Examples of such substances used for modification of surfaces are knownunder the trade names DYNASYLAN™, HYDROSIL™ and PROTECTOSIL™. As aresult, hydrophilic or hydrophobic properties of surfaces or couplinglayers comprising silane oligomers for the attachment of other organicsubstances are obtained.

It is also possible for such plane-parallel platelets of metals such as,for example, aluminium, silver or copper produced in the above manner bythe PVD method and free of troublesome organic residues to be subjectedto subsequent CVD treatment in a further procedure.

For that purpose, the plane-parallel platelets are exposed to a flow ofa layer-forming gas at temperatures of more than 150° C. in a gas-tightcontainer. Such known CVD methods would be impossible, or substantiallymore difficult, if organic residues of coating polymers were still toadhere to the platelets produced by the PVD method, as would be the casewith the described methods according to the prior art. The attempt toremove such substances by vaporisation would result in firmly adheringcracking products remaining behind.

It is furthermore advantageous for the resulting cleaned metal surfacesof plane-parallel platelets produced by the PVD method to be coated withorganic dyes—either directly or by means of a prior wet-chemicalapplication of a coupling layer from 30 to 100 nm thick using methodsknown per se.

Such dyes, for example phthalocyanine, benzimidazolones such asNOVOPERM®, CROMOPHTAL®, GRAPHTOL®, diketopyrrolopyrroles (DPP) andisoindolines having the trade names PALIOTOL® and also perylenepigments, have sufficient thermal stability to be vapour-deposited undera vacuum of from 1 to 10⁻² Pa onto cleaned, plane-parallel plateletsproduced by the PVD method.

In the process, the metal platelets in the form of loose material arecontinuously agitated in a manner known per se until they are encased ina thin layer of those organic materials and also exhibit a pastel colourof greater or lesser intensity depending upon the thickness of the layerapplied. The layers produced in that manner also serve as chemicalprotective layers when such products are used in surface-coatingcompositions comprising aqueous solvents.

The double function of such encased plane-parallel platelets is that ofa coloured reflector and also, when aluminium is used as plateletmaterial, protection thereof against the known evolution of hydrogencaused by reaction of water with aluminium. By that means, unacceptablebubble formation in the surface-coating layer at drying temperatures of50° C. and above is avoided. The double function achieved makes up forthe disadvantage that the coating step requires its own apparatus and,after removal of the separating agent residues by vaporisation has beenperformed, has to be carried out as a final step after mechanicalcomminution of the platelets.

The Examples that follow are intended to further describe the use of theorganic separating agents described herein.

EXAMPLE 4

Production of plane-parallel platelets of aluminium, without furthertreatment, that is to say with a residue of about 5% by weight ofseparating agent on the platelets.

Step 1 Vapour-deposition of a 30 nm layer of phenolphthalein on anendless belt carrier by the method according to U.S. Pat. No. 6 270 840at a vaporisation temperature of 300° C. and under a vacuum of 10⁻² Pa.Step 2 Under the same vacuum and in the same pass: vapour- deposition ofan aluminium layer 40 nm thick. Step 3 After passing through 2 air-lockstages into a vacuum chamber of 5000 Pa: the double layer on the runningbelt is exposed to isopropyl alcohol. The aluminium layer breaks up intofine flakes and comes away from the belt. The suspension is collected invacuo. Step 4 The suspension is discharged from the vacuum chamber, andthe suspension is concentrated to a solids content of 25% bycentrifugation. The procedure is repeated 3 times using fresh isopropylalcohol. Step 5 Further processing by means of comminution and grading,before being processed to form a surface-coating composition.

EXAMPLE 5

Production of surface-cleaned plane-parallel platelets of silver

Step 1 Vapour-deposition of a 30 nm layer of phenolphthalein on anendless belt carrier by the method according to U.S. Pat. No. 6 270 840at a vaporisation temperature of 300° C. and under a vacuum of 10⁻² Pa.Step 2 Under the same vacuum and in the same pass: vapour-deposition ofa silver layer 90 nm thick. Step 3 After passing through 2 air-lockstages into a vacuum chamber of 5000 Pa: the double layer on the runningbelt is exposed to isopropyl alcohol. The silver layer breaks up intofine flakes and comes away from the belt. The suspension is collected invacuo. Step 4 5 kg of the suspension, consisting of 10% silver, 1%dissolved phenolphthalein and 89% isopropyl alcohol, is discharged fromthe vacuum chamber, and the suspension is concentrated to a solidscontent of 50% by centrifugation. Step 5 Comminution of the silverparticles in suspension by means of ultrasound, separation of particlesizes by means of sedimentation. Step 6 Introduction of the suspensioninto a second vacuum apparatus and concentration by evaporation at 5000Pa and a temperature rising from 25 to 60° C. Step 7 Further heating ofthe suspension in the same vacuum apparatus; the wall temperature isincreased to 300° C., with simultaneous recirculation and precipitationof the vaporised phenolphthalein on a cold condenser surface at atemperature of from −196° C. to +25° C. Step 8 The resulting dry silverplatelets freed from contaminants on their surfaces are discharged forfurther processing to form an electrically conductive surface-coatingcomposition.

EXAMPLE 6

Production of plane-parallel copper platelets having a clean surfacewith subsequent coating on all sides with a silane oligomer. Steps 1 to7 are identical to those of Example 5; instead of silver, copper isused.

Step 8 500 g of the copper platelets, cooled to about room temperature,are dispersed under normal pressure in a bath of 10 litres of isopropylalcohol containing 100 g of silane oligomer DYNASYLAN ® Ameo (CAS No.919-30-2*) dissolved therein. 0.55 litre of water is added at 25° C.After about 20 minutes, a hydrophilic, transparent layer of from 20 to30 nm is deposited in the intensively agitated bath. Step 9 Thesuspension is filtered and the coated platelets are dried at 100° C. inthe air stream of a fluidised bed. Step 10 The dry product is suitablefor use as a pigment in decorative copper-tone surface-coatingcompositions. In surface-coating compositions, the product does notfloat on the surface. *: Manufacturer: Degussa-Hüls AG, Rheinfeldenworks

In general, greater addition of water results in a greater amount ofhydrolysis and more rapid but non-uniform coating. In the case ofaluminium, transfer of the produced plane-parallel platelets into thebath should be carried out under a protective gas because of thereactivity of the aluminium.

Organic dyes can be vapour-deposited by the PVD method on theplane-parallel platelets obtained after Step 10.

1. A method for the production of plane-parallel platelets, comprising the steps: a) vapour-deposition, at a pressure below atmospheric pressure, of a separating agent onto a carrier to produce a separating agent layer, b) vapour-deposition, at a pressure below atmospheric pressure, of at least one product layer onto the separating agent layer, and c) dissolution of the separating agent layer in a solvent and production of a suspension in which the at least one product layer is present in the form of plane-parallel platelets, in which method the separating agent is selected from the group consisting of anthracene, anthraquinone, acetamidophenol, acetylsalicylic acid, camphoric anhydride, benzimidazole, benzene-1,2,4-tricarboxylic acid, biphenyl-2,2-dicarboxylic acid, bis(4-hydroxyphenyl)sulfone, dihydroxyanthraquinone, hydantoin, 3-hydroxybenzoic acid, 8-hydroxyquinoline-5-sulfonic acid monohydrate, 4-hydroxycoumarin, 7-hydroxycoumarin, 3-hydroxynaphthalene-2-carboxylic acid, isophthalic acid, 4,4-methylene-bis-3-hydroxynaphthalene-2-carboxylic acid, naphthalene-1 , 8-dicarboxylic anhydride, phthalimide, potassium salt of phthalimide, phenolphthalein, phenothiazine, saccharin, salts of saccharin, tetraphenylmethane, triphenylene, triphenylmethanol, and mixtures thereof.
 2. A method according to claim 1, wherein the product layer encompasses aluminium, iron, copper, silver, zinc, tin, titanium, or mixtures thereof.
 3. A method according to claim 1, wherein the pressure in steps a) and b) is in the range from 10⁻³ to 0.5 Pa and in step c) is in the range from 10 to 2×10⁵ Pa.
 4. A method according claim 1, wherein the solvent encompasses isopropanol, ethyl acetate, butanol, ethanol, petroleum spirit, methyl isobutyl ketone, methyl ethyl ketone, perchloroethylene, or mixtures thereof.
 5. A method according to claim 1, wherein the separating agent is vaporised from a mixture comprising a ceramic material in granule or powder form, the ceramic material remaining unvaporised as a residue.
 6. A method according to claim 1, wherein the separating agent is vaporised from a crucible having a ceramic or glass surface or a quartz surface.
 7. A method according to claim 1, wherein the separating agent is fed in to the vaporisation source, in vacuo, continuously or at time intervals.
 8. A method according to claim 1, wherein, before the separating agent layer is dissolved, there is vapour-deposited a combination of layers consisting of more than two layers, a further separating agent layer being located between every two product layers.
 9. A method according to claim 1, wherein the separating agent layer has a thickness of from about 10 nm to about 200 nm and the product layer has a thickness of from about 10 nm to about 500 nm.
 10. A method according to claim 1, comprising the following further step: d) removal of separating agent residues by vaporisation at up to 350° C. and under a vacuum of from 10 to 10⁻³ Pa.
 11. A method according to claim 10, comprising the following further step: e) application of hydrophobic layers, hydrophilic layers or coupling layers comprising silane oligomers to the cleaned surfaces—produced by vaporisation in step d)—of the plane-parallel platelets.
 12. A method according to claim 10, comprising the further step of simultaneous agitation of loose material obtained in step d) along with vapour-deposition of organic dyes by a physical vapor deposition method.
 13. A method according to claim 11, comprising the further step of subjecting agitated loose material obtained in step a) to vapour-deposition of organic dyes by a physical vapor deposition method. 